The present invention is related to coded data generation or conversion and, more particularly to a method and apparatus for converting a non-return to zero (NRZ) signal to a return to zero (RZ) signal.
Modem optical data communication networks often require that data transmitted through the optical communication network be encoded in a return-to-zero (RZ) format. Since a majority of digital electronic data is encoded using a non-return-to-zero (NRZ) format, the digital electronic signals must be converted from NRZ format to RZ format to be transmitted within an optical communication network.
Existing high speed RZ optical network transmitters use various methods of converting NRZ encoded data received from a multiplexer into RZ encoded data before it is sent over an optical fiber. The most common method uses two optical amplitude modulators. The first optical modulator is driven by a clock signal and produces a stream of RZ xe2x80x9cones.xe2x80x9d The second optical modulator is driven by NRZ data. The second optical modulator gates the stream of RZ xe2x80x9conesxe2x80x9d to produce RZ data. Although simple, the multiple optical modulators make this technique of generating RZ data expensive.
U.S. Pat. No. 5,625,722, entitled xe2x80x9cMETHOD AND APPARATUS FOR GENERATING DATA ENCODED PULSES IN RETURN-TO-ZERO FORMAT,xe2x80x9d discloses an alternate method of generating RZ signals for transmission in an optical network. A laser is used to generate a continuous light stream which is the first input of a modulator. The NRZ signal gates the continuous light stream by allowing the light stream to output an optical signal only when the NRZ signal transitions from predetermined levels. This method of signal conversion is also more expensive than electrical conversion of the NRZ signal to a RZ format. U.S. Pat. No. 5,625,722 is hereby incorporated by reference into the specification of the present invention.
A third known method of electrical NRZ-to-RZ conversion uses high-speed digital logic integrated circuits (ICs). A NAND gate that has NRZ data and a clock signal as it two inputs will produce inverted RZ data as its output. These ICs, although much cheaper than optical modulators, are also expensive.
What is needed is an efficient and cost effective method and apparatus for electrically converting a NRZ signal into a RZ signal.
The present invention is intended to provide a low cost method and apparatus for converting a NRZ signal with a synchronous clock signal to a RZ signal.
In one embodiment, an incoming clock signal is phase aligned to an incoming NRZ signal. The NRZ signal and the clock signal are then summed to produce a combined signal. The combined signal is then passed across a Pseudomorphic High Electron Mobility Transistor (PHEMT). The PHEMT is biased near pinchoff such that only those portions of the clock signal that were combined with the digital xe2x80x9conesxe2x80x9d of the NRZ signal cause an output signal on the PHEMT. Portions of the clock signal that were combined with the digital xe2x80x9czeroesxe2x80x9d of the NRZ signal are clipped. The resultant signal is an inverted RZ signal.